Mold assembly and method for encapsulating semiconductor device

ABSTRACT

A mold assembly including: a first mold half; a second mold half relatively movable with respect to the first mold half; and a thin film disposes between the both mold halves and in contact with the surface of a semiconductor chip. Because of the contact between the edges of the surface of the semiconductor chip and the thin film, the portion of the semiconductor chip at which the burr is liable to be generated is protected and no burrs are generated.

BACKGROUND OF THE INVENTION

[0001] (a) Field of the Invention

[0002] The present invention relates to a mold assembly and a method forencapsulating a semiconductor device, more in detail to the moldassembly and the method capable of encapsulating a semiconductor chipwith its surface exposed on the surface of a package.

[0003] (b) Description of the Related Art

[0004] A conventional method for molding the package of semiconductorchips includes two techniques, that is, a potting technique and atransfer technique.

[0005] In the conventional potting technique for encapsulating asemiconductor chip with the bottom surface thereof exposed on thesurface of a package, a potting apparatus shown in FIG. 1 is used. Theapparatus shown therein supplies a tape 102 reinforced with a frame 101and moving on the sample stage of a potting device, and applies liquidresin thereto by moving a syringe 103 filled with the liquid resin alongthe tracks or locus specified by input data. The movement is effectedeither by the X-Y-Z-direction movement of the syringe itself or theX-Y-direction movement of the sample stage. The liquid resin in thesyringe 103 is ejected for application by controlling the pressure ofair in a dispenser unit 104.

[0006] The liquid resin is expensive and its handling including storageand use thereof is difficult. In addition, the amount of the ejectedliquid resin depends on the viscosity of the resin, the pressure of thedispenser unit and a period of application time. The diversity of theamount of the ejected liquid resin generates an unevenness of thethicknesses in the molds. The planarity or flatness of the sample stagealso generates the unevenness of the thicknesses in the molds.

[0007] Further, in the potting technique described above, somedeficiencies exist mainly because no pressure can be applied during theapplication of the resin. The adherence between the semiconductor chipand the resin in the potting technique is weaker due to the applicationof no pressure than that in the transfer technique, and peeling-off ofthe resin at the interface is liable to occur. Further, in the pottingtechnique, in order to prevent a void, the syringe slowly moves to applythe liquid resin, and accordingly a cycle of application time is longand its throughout is lower than that of the transfer technique. Whenthe solder balls on the bottom surface of the tape are disposed outsideof the chip (fan-out structure), an inconvenience for the balls may begenerated such as co-planarity deficiency and ball missing due to thelower strength of the resin.

[0008] In order to remove the above deficiencies in the pottingtechnique, the transfer technique for encapsulating semiconductor chipsin a package by the molding is employed. However, the current transfertechnique involves other problems such that (1) burrs are generated atan exposed portion and a manufacturing cost rises due to an addition ofa step of removing the burrs, and (2) the chip is directly clamped withthe mold to generate damages in the chip.

SUMMARY OF THE INVENTION

[0009] In view of the foregoing, an object of the present is to providea mold assembly and a method for encapsulating a semiconductor device inwhich the molding for encapsulation preferably used in the transfertechnique provides no burrs.

[0010] The present invention provides, in a first aspect thereof, a moldassembly for molding a semiconductor chip including: first and secondmold halves disposed for relative movement with respect to each other ina first direction for closing and opening the mold assembly; and a moldreleasing member disposed between the semiconductor chip and one of thefirst and second mold halves, the mold releasing member having a firstsurface in contact with the semiconductor chip during a closed state ofthe mold assembly, the mold releasing member having a property of beingreleased from one of the first and second mold halves after molding by aresin, the first surface being larger than a surface of thesemiconductor chip in contact with the first surface.

[0011] The present invention provides, in a second aspect thereof, amethod for molding a semiconductor chip in a mold assembly having firstand second mold halves disposed for relative movement with respect toeach other, the method including the steps of: sandwiching a moldreleasing member and the semiconductor chip between the first mold halfand the second mold half, the mold releasing member having a property ofelastically deforming, the releasing member and one of the first andsecond mold halves defining a cavity for receiving the semiconductorchip therein; and injecting resin in the cavity by using a firstpressure lower than a pressure, which allows the mold releasing memberto elastically deform, in a closed state of the mold assembly to therebymold the semiconductor chip.

[0012] In accordance with the first and the second aspects of thepresent invention, the use of the mold releasing member can prevent thegeneration of burrs because the edges of the surface of thesemiconductor device is in contact with the film.

[0013] The above and other objects, features and advantages of thepresent invention will be more apparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

[0014]FIG. 1 is a vertical elevational view showing a potting apparatusused in a conventional method.

[0015]FIG. 2 is a vertical cross-sectional view showing an encapsulationmold for a semiconductor device in accordance with an embodiment of thepresent invention.

[0016]FIG. 3 is a top plan view showing a semiconductor chip to beencapsulated.

[0017]FIG. 4 is a cross-sectional view showing the chip of FIG. 3.

[0018]FIG. 5 is a cross-sectional view showing the encapsulation mold ofFIG. 2 when it is fastened.

[0019]FIG. 6 is an enlarged vertical cross-sectional view of the moldfor showing the function thereof.

[0020]FIG. 7 is a vertical cross-sectional view showing an encapsulationmold for semiconductor device in accordance with another embodiment.

PREFERRED EMBODIMENTS OF THE INVENTION

[0021] Now, the present invention is more specifically described withreference to accompanying drawings.

[0022] First Embodiment

[0023] A mold assembly for encapsulating a semiconductor device includesa transfer assembly mold having a fixed, upper mold half 11 and amovable, lower mold half 12. The lower mold half 12 includes a mold bodymember 13 having a top, shallow concave surface 14 and a chip holder 15engaged and received in the concave surface 14.

[0024] The upper mold half 11 includes a holder 16 and a thrust mold 17.The thrust mold 17 is guided by the holder 16 for slidable movement withrespect to the holder 16 in a direction of the relative movement betweenthe upper mold half 11 and the lower mold half 12. An elastic member 18,preferably a coned disc spring, is disposed between the bottom surfaceof the holder 16 and the top surface of the thrust mold 17. A vacuumspace 19 is formed around the thrust mold 17. A thin film 21 is disposedbetween the upper mold half 11 and the lower mold half 12. The film 21has a property of being elastically deformed wherein it reduces thethickness thereof to some extent when compressed. The film 21 is suckedfrom a space between the holder 16 and the thrust mold 17 to beattracted to the upper mold half 11.

[0025] The chip holder 15 shown in FIGS. 3 and 4 includes a frame 22, awired pattern layer 23 having an adhesive layer and solder balls, and aplurality of chips 24 arranged on the wired pattern layer 23. As shownin FIG. 4, the top adhesive layer of the wired pattern layer 23 isbonded onto the bottom surface of the frame 22. The plurality of chips24 are bonded to the wired pattern layer 23 through an opening 25 formedin the frame 22.

[0026] The chip holder 15 is mounted in the lower mold half 12 as shownin FIG. 2. The wired pattern layer 23 of the chip holder 15 is engagedwith and received in the concave surface 14. The concave surface 14 haschannels formed in accordance with the shapes of the chip holder 15 andthe wired pattern layer 23.

[0027] The mold assembly is closed by ascending the lower mold half 12to be in close contact with the upper mold half 11 as shown in FIG. 5.An enclosed cull 26 is formed between the upper mold half 11 and thelower mold half 12 and solid resin is supplied in the cull 26. A plunger27 is fitted in the lower mold half 12, and a runner 28 communicatedwith the cull 26 is formed in the upper mold half 11. The bottom of therunner 28 is open at the bottom surface of the holder 16.

[0028] The film 21 is automatically supplied and discharged by a knownmechanism not shown in the drawings. The mechanism may include one ormore supply rollers for supplying the film from a wound film roll andone or more discharge rollers for discharging the used film bywinding-up after the transfer molding. The rollers are disposed at bothends of the upper mold half 11, and the supply and the discharge of thefilm are simultaneously conducted by moving the film by the plurality ofrollers.

[0029] In the closed state of the mold assembly shown in FIG. 5, thebottom peripheral surface of the holder 16 and the bottom surface of thethrust mold 17 are in contact with the top surface of the film to form asingle plane.

[0030] The resin 31 in the cull 26 is melted with heat, supplied to thecavity through the runner 28 by the pressure applied by the plunger 27,and then supplied to the opening 25 of the chip holder 15.

[0031] The closed state is shown more in detail in FIG. 6 in which thefilm 21 is pressed between the resin 31 and the thrust mold 17 to beelastically deformed and a part of the film 21 is compressed by theplurality of the chips 24. Since the melted resin 31 does not enter intothe space between the top surface of the chip 24 and the bottom surfaceof the thrust mold 17, no burrs are generated on the edges of the topsurface of the chips 14.

[0032] The thrust mold 17 of the upper mold half 11 resides in afloating state in order to prevent damages, such as a crack in the chip24, generated due to abnormal pressure rise between both the mold halvesand unevenness of the chip thicknesses. The setting of the pressure bythe thrust mold 17 at this time is higher than an injection moldingpressure so that no adverse influence by the injection molding pressureis produced. The pressure determined in this manner effectively preventsthe generation of the burrs mentioned before. While a preferableeffective injection pressure is 100 kg/cm², a preferable setting of thepressure by the thrust mold 17 is 200 kg/cm².

[0033] The combination of the film 21 and the pressure by the thrustmold 17 enables the uniform pressing of all the chips regardless of theunevenness of the chip thicknesses, thereby effectively preventing thegeneration of the burrs on the surface.

[0034] In another embodiment shown in FIG. 7, a chip holder 15 a isdifferent from the chip holder, 15 in FIG. 3. A chip 24 is fixed onto alead frame 30, and a bonding wire 29 connects the lead frame 30 and theelectrode of the chip 24.

[0035] A heat spreader 32 is opposed to the bottom surface of the leadframe 30. An upper cavity 34 into which the chip 24 and the lead frame30 enter is formed at the bottom part of the upper mold half 11, and alower cavity 35 is formed at the corresponding part of the lower moldhalf 12. In the present embodiment, the heat spreader 32 in place of thechip 24 is exposed and encapsulated in the lower cavity 35.

[0036] The film 21 a is supplied between the lower mold half 12 and thechip holder 15 a, and the mold assembly is closed as shown in FIG. 7.The film 21 a is pressed between the bottom surface of the lower cavity35 and the bottom surface of the heat spreader 32. The side surfaces ofthe heat spreader 32 are covered with the resin supplied from the runner28 while exposing the bottom surface of the heat spreader 32.

[0037] Also in the present embodiment, the generation of the burrs atthe edges of the surface of the heat spreader 32 is prevented. In thiscase, the burr generation is prevented by properly pressing the film 21a on the heat spreader 32 by the injection pressure of the resin. Thefloating mechanism of the elastic member 18 in FIG. 2 is unnecessary inthe present embodiment because the chip 24 is not oppressed.

[0038] Since the above embodiments are described only for examples, thepresent invention is not limited to the above embodiments and variousmodifications or alternations can be easily made therefrom by thoseskilled in the art without departing from the scope of the presentinvention.

What is claimed:
 1. A method for molding a semiconductor chip in a moldassembly having first and second mold halves disposed for relativemovement with respect to each other, said method comprising the stepsof: sandwiching a mold releasing member and the semiconductor chipbetween said first mold half and said second mold half, the moldreleasing member having a property of elastically deforming, thereleasing member and one of said first and second mold halves defining acavity for receiving the semiconductor chip therein; ands injectingresin in the cavity by using a first pressure lower than a pressure,which allows the mold releasing member to elastically deform, in aclosed state of the mold assembly to thereby mold the semiconductorchip.
 2. The method as defined in claim 1, wherein the mold releasingmember is a film having a substantially uniform thickness.
 3. The methodas defined in claim 1, further comprising the step of releasing the moldreleasing member from the one of the first and second mold halves. 4.The method as defined in claim 1, wherein the closed state is achievedby elastically holding at least one of the first and second mold halves,and the first pressure is lower than a pressure that allows the moldassembly to open from the closed state.